The reason why Sony developed this DC Phase Linearizer back ~20years ago is because Class D bass response is quite different from traditional Class A and Class A/B amps. Most music listeners are more used to a more slower bass beat of the traditional amps. If you listen carefully with DSP off, Class D sounds more speed orientated with less decay when it comes to bass notes.
DC Phase Linearizer makes it more easy to notice bass notes as there is added decay or slowness to the beat.
Type A tends to be more upfront/stronger, making bass notes sounding more foreground in the music. Bass notes sounds more "Thudddd" like it's presentation.
Type B makes bass notes fall behind the vocals, sounding more laid-back/background like. Bass notes sound more "Dunnn" like in it's presentation.
As for low, standard and high:
Low has an emphasis on sub bass
Standard has an emphasis on mid bass
High makes bass appear as one body of note.
Which mode to use, off or type A or B, low or standard or high:
It all depends on your listening preferences. You can use this DSP as another EQ to tune the bass response of your audio gear, you may find a setting that makes your headphones sound better to your ears.